1. Field of the Invention
The present invention relates to color computer graphics and more particularly to color gamut mapping in which source device colors not reachable by a destination device are mapped to other colors that are reachable by the destination device.
2. State of the Art
Color matching algorithms are intended to present information on one medium so that it appears the same as when presented on a different medium. For Cathode Ray Tube (CRT) color displays, the CIE (Commission Internationale de l'Eclairage) has established standards which allow all displays to present color in the same manner, even though different colors are used to generate the image. However, since printing is a subtractive process (CRTs using an additive process), it becomes difficult to tell a printer what amounts of its subtractive primaries to use to present the same color when told the additive primaries of the CRT.
Neugebauer, Electronics For Imaging, Kodak, and numerous others have attempted to convert from the color representation on a CRT to a color specification on a printer. The general consensus is that an interpolated table is an adequate method for converting from the CRT color space to the printer's color space. However, if one measures the colors available to a CRT and those available to a color printer, one finds that a small amount of colors which are printable cannot be displayed on the CRT, while many colors which may be displayed on the CRT cannot be printed, especially green, yellow, red, and magenta. The color range available to an imaging device is referred to as its color gamut, and this non-overlapping nature of the color gamuts between devices is known as gamut mismatch, and the colors which a source device can image but the destination device cannot image are known as out of gamut colors.
Conventionally, the general rule for mapping from a source device (such as a CRT) to a destination device (such as a color printer) is that all colors which are available to the destination device should print the source device's color as closely as possible; when printing out of gamut colors, hue angle, saturation, and lightness of the source device should be preserved as closely as possible.
However, when this algorithm is implemented, users do not like the results, especially when attempting to present monitor saturated colors such as yellow, green, red, and magenta. Usually, the yellow is the most objectionable, because most CRT yellows are more green than the printer's yellow. The result is that when saturated CRT yellow is requested, the color matching algorithm maps this color to white or a washed-out green. This typically isn't what the user had in mind when requesting saturated yellow, although this is a correct color science representation of that color.
Assuming that images to be printed will typically consist of a combination of natural images and computer generated images, ideally the natural images would map to the closest printable colors, and the computer generated images would map to the saturated printer colors. However, there is usually no way to tell the source of a color specification.
What is needed, then, is a method whereby natural images may be printed to match as closely as possible the color seen on a CRT while presenting saturated CRT colors as saturated printer colors.